Field of the Invention
Embodiments of the invention relate to an organic light emitting diode display. More particularly, embodiments relate to an organic light emitting diode display capable of sensing electrical characteristics of a driving element.
Discussion of the Related Art
An active matrix organic light emitting diode (OLED) display includes organic light emitting diodes (OLEDs) capable of self-emitting light, by itself and has advantages of a fast response time, a high emission efficiency, a high luminance, a wide viewing angle, and the like.
The OLED serving as a self-emitting element includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emission layer EML and form excitons. As a result, the emission layer EML generates visible light.
The OLED display arranges pixels each including the OLED in a matrix form and adjusts a luminance of the pixels based on gray levels of video data. Each pixel includes a driving element, i.e., a driving thin film transistor (TFT) controlling a driving current flowing in the OLED depending on a voltage Vgs between a gate electrode and a source electrode of the driving TFT. Electrical characteristics (including a threshold voltage, a mobility, etc.) of the driving TFT may be deteriorated with the passage of driving time, causing a characteristic variation in the pixels. In other words, a variation in the electrical characteristics of the driving TFTs of the pixels results in a luminance variation in the pixels, to which the same video data is applied. Hence, it is difficult to implement a desired image.
An external compensation method is known to compensate for the variation in the electrical characteristics of the driving TFTs. The external compensation method senses change in the electrical characteristic of the driving TFT through a sensing unit and modulates digital video data through an external circuit by an amount of change in the electrical characteristic of the driving TFT. The external compensation method has an advantage in that a pixel circuit is not complicatedly configured. A method for sensing the change in the electrical characteristic of the driving TFT through the sensing unit of the external compensation method includes a voltage sensing method and a current sensing method.
The voltage sensing method stores a current flowing in the driving TFT, as a voltage, in a line capacitor of a sensing line and then senses the voltage through the sensing unit. However, because a line capacitance of the sensing line is very large, it takes a long time to pull in the current at a voltage level the sensing unit can sense. Furthermore, because the line capacitance varies depending on a display load of the display panel, it is difficult to obtain an accurate sensing value through the voltage sensing method.
On the other hand, as shown in FIG. 1, the current sensing method is configured so that a sensing unit includes a current integrator CI and directly senses a current flowing in the driving TFT. Therefore, the current sensing method can perform the low current and high-speed sensing and also can perform the relatively accurate sensing because an influence of the display load decreases. In the current sensing method, the current flowing in the driving TFT of the pixel is applied to the current integrator CI through the sensing line and is changed into a voltage through an integration process of the current integrator CI. The voltage changed from the current passes through a sample and hold unit SH and is transferred to an analog-to-digital converter (ADC). The ADC converts the voltage into a digital sensing value.
However, because a pixel current (i.e., a source-to-drain current Ids of the driving TFT) Ipix, that generally becomes a target of the sensing, is very small, the current sensing method using the current integrator CI is weak to a noise of an external power source. The noise is generated by a variation in a reference voltage VREF applied to a non-inverting input terminal (+) of an amplifier AMP constituting the current integrator CI, a variation in a reference voltage EVREF applied to one side of a sampling capacitor C of the sample and hold unit SH, a difference between noise sources of sensing lines connected to an inverting input terminal (−) of the amplifier AMP, etc. Because the noise is amplified inside the current integrator CI and is reflected on an integration value, the noise may distort the sensing result as shown in FIG. 2. A first sensing value of FIG. 2, with which the noise is mixed, reduces a sensing performance, resulting in a reduction in a compensation performance.